1. Field of the Invention
The present invention relates to a display device provided with a substrate formed of arrayed thin film cathodes for emitting electrons from a top electrode side when a voltage is applied between a base electrode and the top electrode, which thin film cathodes comprises the base electrode, the top electrode, and an electron acceleration layer such as insulator held between these electrodes, and provided with a phosphor screen.
2. Description of the Prior Art
Thin film cathode basically has the structure comprising laminated three thin films, namely a top electrode, electron acceleration layer, and base electrode, and a voltage is applied between the top electrode and the base electrode to thereby emit electrons from the surface of the top electrode into the vacuum. For example, a MIM (Metal-Insulator-Metal) type thin film cathode comprising a laminate of a metal, an insulator, and a metal, a MIS (Metal-Insulator-Semiconductor) type thin film cathode comprising a laminate of a metal, an insulator, and a semiconductor, and a thin film cathode comprising a laminate of a metal, an insulator-semiconductor, and a metal are exemplified as the thin film cathode. MIM type (for example, Japanese Patent Laid-open No. Hei07-65710), MIS type of MOS type (J. Vac. Sci. Technol, B11(2) p. 429-432 (1993)), the metal/insulator-semiconductor/metal type of HEED type (high-efficiency-electro-emission-device,Jpn.J.Appl. Phys.,vol. 36, p L939), EL type (Electroluminescence, OYO BUTURI vol. 63, No. 6, p. 592), porous silicone type (OYO BUTURI vol. 66, No. 5, p. 437) have been reported.
The operation mechanism of a MIM type thin film cathode is exemplarily shown in FIG. 2. A driving voltage Vd is applied between the top electrode 13 and the base electrode 11 to form an electric field of approximately 1 to 10 MV/cm in the electron acceleration layer 12. Thereby, electrons in the vicinity of the Fermi level in the base electrode 11 pass through the barrier due to the tunnel phenomenon and are injected into the electron acceleration layer 12 and conduction band of the top electrode 13 as hot electrons. Though hot electrons are scattered in the electron acceleration layer and the top electrode 13 and lose the energy, partial hot electrons having the energy higher than the work function of the top electrode 13 are emitted into the vacuum 20.
Other thin film cathodes accelerate electrons and emit electrons through a thin top electrode 13, that operation is common for all the other thin film cathodes.
A thin film cathode having a matrix structure comprising a plurality of top electrodes and a plurality of base electrodes that are disposed orthogonally each other can generate an electron beam from an arbitrary position, and can be utilized as the electron source of a display device. It has been observed that a MIM (Metal-Insulator-Metal) structure comprising Auxe2x80x94Al2O3xe2x80x94Al laminate emits electrons.
Because a thin top electrode is used for a thin film cathode array, usually a top bus electrode that is served as the current feed line is added for applying it to a display device. At that time, connection between a top bus electrode and a top electrode is formed very carefully so that the thin top electrode is not broken. To process a thin top electrode without damage of an electron acceleration layer and without contamination of the top electrode with resist, it is effective that an inter layer insulation film having an electron emitting aperture is formed on the top bus electrode and the top electrode is cut into pixel pieces by use of the interlayer insulation film as a mask.
The inventors of the present invention has proposed a method (Japanese Patent laid-open No. 2001-256907) in which, as shown in FIG. 3, a top bus electrode comprises two layers, namely a thin top bus electrode under-layer 16 and a thick bus electrode upper-layer 17, electrical contact is secured by the thin top bus electrode under-layer 16 without breakage of the top electrode 13, the top bus electrode upper-layer 17 is formed so as to support the overhang of the interlayer insulation film 18, and the step formed as the result of the configuration is served to separate the top electrode 13 as the structure of the top bus electrode that satisfies the two conditions. In detail, a thin W film is used as the top bus electrode under-layer 16, a thick Al alloy layer is used as the top bus electrode upper-layer 17, and an Si3N4 film or SiO2 film is used as the interlayer insulator 18.
When the thin film cathode is applied to a display, a thin film cathode is inevitably subjected to frit glass sealing at a temperature as high as 400xc2x0 C. or higher for glass bonding to a phosphor screen. The thin film cathode having the conventional structure can be involved in two problems when it is subjected to the high temperature process. At first, one problem is oxidation of a thin top bus electrode under-layer 16.
It is preferable for low cost manufacturing that frit glass sealing is carried out in an atmospheric environment for baking organic binder contained in the frit glass paste and for omitting equipment and process that are necessary for gas replacement. However, the electrode material is oxidized. Even if the high temperature process is carried out in an inert gas atmosphere for preventing oxidation, oxidation of an electrode cannot be prevented due to oxygen contained in the inert gas though the content is very little. The top bus electrode under-layer 16 having the conventional structure is involved in the problem of insufficient oxidation resistance. For example, though W is rather an oxidation resistant material, a W film having a thickness as thin as approximately 10 nm is almost oxidized entirely when it is heated at a temperature of 400xc2x0 C. or higher to result in increased resistance value of the sheet, and the electric contact with the top electrode 13 becomes poor. On the other hand, though a W film having a thickness as thick as 20 nm or thicker causes suppression of diffusion of oxidation seeds of the surface oxidized film and the oxidation of the whole film is prevented to thereby maintain the low resistance, the step becomes large to reduce the reliability of the electrode connection.
The other problem relates to breaking of a thin top bus electrode under-layer 16 due to thermal stress caused from high temperature treatment for frit glass sealing. The top bus electrode under layer 16 is deformed due to the stress caused from the difference in thermal expansion coefficient between the top bus electrode upper-layer 17 and the interlayer insulation film 18 and due to the stress caused from densification of the top bus electrode upper-layer 17 and the interlayer insulation film 18 when the thin film cathode is subjected to high temperature heat treatment cycle for frit glass sealing. Because the top bus electrode under-layer 16 is very thinner than the top bus electrode upper-layer 17 and interlayer insulator 18, the top bus electrode under-layer is apt to break.
It is an object of the present invention to provide a thin film cathode that is capable of maintaining high electric contact reliability with a top electrode 13 by preventing oxidation of a top bus electrode under-layer 16, or by preventing lowering of the reliability of electric connection with the top electrode 13 due to a step in the case that a top bus electrode 16 having a thick thickness is used to prevent the oxidation of the top bus electrode under-layer 16, and by preventing the breakage of electric connection with the top bus electrode under-layer 16 due to thermal stress even though the thin film cathode is subjected to high temperature frit glass sealing treatment. Thereby, a highly reliable display device can be realized at a high manufacturing yield.
This object of the present invention is realized by providing a display device having a substrate comprising a base electrode, a top electrode, and an electron acceleration layer disposed between the base electrode and the top electrode, which substrate is formed of arrayed thin film cathodes that emit electrons from the top electrode side by applying a voltage between the base electrode and the top electrode, and a phosphor screen, wherein the arrayed thin film cathodes have a top bus electrode served as a current feed line to the top electrode, which top bus electrode is tapered so that the film thickness decreases toward an electron emission portion side, and have an overhang structure that separates the top electrode into individual cathodes.
The overhang structure comprises a laminate of films consisting of two or more different materials respectively. The top bus electrode and the overhang structure are realized by using a laminate constituted of a first metal layer (top bus electrode), second metal layer, and insulator in the order from the top bus electrode side, or a laminate constituted of a first metal layer (top bus electrode), insulator, and second metal layer in the order from the top bus electrode side, or otherwise a laminate constituted of a first metal layer (top bus electrode), a first insulator, and second insulator in the order from the top bus electrode side.
The top bus electrode and the overhang structure are realized by employing the combination of a material used for the top bus electrode that is etched slowest and a material used for the film of the overhang laminate film disposed on the side of the top bus electrode that is etched fastest with the same etchant is employed for the top bus electrode and the overhang structure or by employing the combination of materials of top bus electrode and the overhang laminate film that are etched selectively.